linux-kernel - RE: [PATCH net-next] x86: bpf_jit_comp: secure bpf jit against spraying attacks

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Date:	Mon, 20 May 2013 09:51:59 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric Dumazet" <eric.dumazet@...il.com>,
	"David Miller" <davem@...emloft.net>
Cc:	"netdev" <netdev@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH net-next] x86: bpf_jit_comp: secure bpf jit against spraying attacks

> hpa bringed into my attention some security related issues
> with BPF JIT on x86.
> 
> This patch makes sure the bpf generated code is marked read only,
> as other kernel text sections.
> 
> It also splits the unused space (we vmalloc() and only use a fraction of
> the page) in two parts, so that the generated bpf code not starts at a
> known offset in the page, but a pseudo random one.
...
> +static struct bpf_binary_header *bpf_alloc_binary(unsigned int proglen,
> +						  u8 **image_ptr)
...
> +	/* insert a random number of int3 instructions before BPF code */
> +	*image_ptr = &header->image[prandom_u32() % hole];
> +	return header;
> +}

Hmmm.... anyone looking to overwrite kernel code will then start
looking for blocks of 0xcc bytes and know that what follows
is the beginning of a function.
That isn't any harder than random writes.

Copying a random part of .rodata might be better - especially
if you can find part of .rodata.str*.

	David